Overheating. (don't think so though, unless airflow from fans now hindered by new, bigger card.)
Game installed when Old GFX card installed, and now trying to use settings/drivers relevant to old card, when old card no longer there?
First, I would make sure the new card properly plugged in, and that the fitting of it did not knock anything else lose.
Next, Check device manager for conflicts.
Copy saved game folder to safe place, and uninstall game, and re-install game?
Direct X also installs files specific to GFX card, and now the card has changed. Uninstall/repair/reinstall Direct-X?
Windows also installs GFX card specific files. Did you remove all drivers for old card before fitting new one? (how to do this seems to vary depending on the old card)